The present invention relates to a magnetic head for magneto-optic recording wherein an information signal is recorded on a magneto-optic recording medium, and also to a magneto-optic recording apparatus which adopts the magnetic head.
There has heretofore been known a magneto-optic recording apparatus which records an information signal in such a way that a magnetic field modulated by the information signal is applied to a magneto-optic recording medium, and that a light beam is focused to irradiate the recording medium therewith. Such a magneto-optic recording apparatus includes a magnetic head for applying the magnetic field to the magneto-optic recording medium, and an optical head for irradiating the recording medium with the light beam. Usually, the magnetic head of this sort is configured of a core which is formed with a magnetic pole in the shape of a pillar opposing the magneto-optic recording medium perpendicularly, and a coil which is mounted around the magnetic pole. In addition, the core is made of a soft magnetic material such as ferrite, for example, an Mn--Zn ferrite (containing Fe.sub.2 O.sub.3, MnO and ZnO as principal components) disclosed in the official gazette of Japanese Patent Application Laid-open No. 5-166630.
Here, as the permeability of the soft magnetic material making the core of the magnetic head is higher, the magnetic head can generate a stronger magnetic field for a predetermined current supplied to the coil. That is, the magnetic head has a higher efficiency for generating the magnetic field. Also the above official gazette teaches examples of various ferrites whose relative permeabilities .mu. are 1090 to 2560 inclusive at a frequency of 1 MHz. In the point of heightening the efficiency of the magnetic head for generating the magnetic field, it is desirable to make the core out of the ferrite of high relative permeability .mu. in this manner. However, that appropriate range of relative permeabilities .mu. in which influences on the other characteristics of the magnetic head are also considered has not hitherto been sufficiently studied.
Meanwhile, in recent years, the modulation of the magnetic field at a higher frequency has become necessary in the magneto-optic recording apparatus stated before, in compliance with a requirement for heightening the recording speed of the information signal. Nevertheless, as the modulation frequency of the magnetic field is set higher, power loss in the magnetic head increases more, with the result that the magnetic head generates heat to raise its temperature more. The main cause of such power loss in the magnetic head is high-frequency loss in the core. When the temperature of the magnetic head, especially the core, rises due to the heightened modulation frequency of the magnetic field, problems occur in that the magnetic characteristics of the magnetic head degrade, and so forth. In particular, the saturation magnetic flux density of the ferrite making the core decreases with a rise in the temperature. Therefore, when the modulation frequency in the magnetic field is heightened, the saturation magnetic flux density finally becomes equal to a magnetic flux density in the interior of the core. Besides, when the modulation frequency is heightened still more, the magnetic flux density in the interior of the core decreases with that lowering of the saturation magnetic flux density which is ascribable to the rise in the temperature. In consequence, the magnetic field which is generated by the magnetic head also is decreased to render normal recording of the information signal impossible. Such a problem becomes more serious particularly in a case where the maximum modulation frequency of the magnetic field (the inverse number of double the minimum time interval of the inversion of the magnetic field) is set at 5 MHz or above.
The present invention has for its object to provide a magnetic head for magneto-optic recording in which a magnetic field to be generated by a core does not decrease even at high frequencies.
As the result of making a study on the above problem, the inventors determined that, when the core of a magnetic head is made of a ferrite having the appropriate characteristics of a relative permeability .mu. and has its magnetic pole set at appropriate dimensions, it is possible to diminish the high-frequency loss of the core which is the cause of the heat generation of the magnetic head and to generate a magnetic field of satisfactory strength forming no hindrance to the recording of an information signal. Thus, the problem involved in the prior art as stated before can be favorably solved.
Concretely, the problem of the prior art can be favorably solved by constructing a magnetic head for magneto-optic recording, having a core which includes a magnetic pole in the shape of a pillar perpendicular to a magneto-optic recording medium; and a coil which is mounted around the magnetic pole; where the core is made of a soft magnetic ferrite whose relative permeability at a frequency of 100 kHz has a real part .mu.1' being at least 5 and at most 700 and whose resonance frequency fr is at least 2.1 MHz, and that .alpha..ltoreq.3 holds in a case where a height H and an average cross-sectional area S of the magnetic pole of the core are related as H/S=.alpha..
Alternatively, a similar effect can be attained when the magnetic head for magneto-optic recording is constructed so that the core is made of a soft magnetic ferrite whose relative permeability at a frequency of 100 kHz has a real part .mu.1' being at least 5 and at most 700 and also has an imaginary part .mu." becoming a maximum at a frequency fp of at least 8 MHz, and that .alpha..ltoreq.3 holds in a case where a height H and an average cross-sectional area S of the magnetic pole are related as H/S=.alpha..
Yet alternatively, a similar effect can be attained when the magnetic head for magneto-optic recording is constructed so that the core is made of a soft magnetic ferrite whose relative permeability at a frequency of 100 kHz has a real part .mu.1' being at least 5 and at most 700 and whose relative permeability in a frequency range of above 100 kHz to at most 5 MHz has a real part being at least 0.5 .mu.1' and at most 1.5 .mu.1", and that .alpha..ltoreq.3 holds in a case where a height H and an average cross-sectional area S of the magnetic pole are related as H/S=.alpha..
More desirably, it is permitted to generate a magnetic field at a still higher efficiency, when the magnetic head for magneto-optic recording is constructed so that the real part .mu.1' of the relative permeability of the ferrite making the core, at the frequency of 100 kHz, satisfies the following equation in accordance with the quantity .alpha.: EQU 10.sup.1.45+0.343.alpha..ltoreq..mu.1'.ltoreq.10.sup.1.93+0.480.alpha.
Also effective for solving the problem stated before is that a Curie temperature Tc of the ferrite making the core is at least 200.degree. C., while a saturation magnetic flux density Bs thereof at a temperature of 25.degree. C. is at least 3500 Gausses.
Also effective for solving the problem stated before is that a relative permeability of the ferrite making the core, in a temperature range of at least 20.degree. C. to at most 80.degree. C., has a real part .mu.' of plus temperature coefficient.
The magnetic characteristics mentioned above can be realized particularly by an Ni--Zn ferrite of cubic system or a ferrite of hexagonal system.